The mechanisms involved in the sensitization of a system to ionizing radiation through the use of compounds such as bromouracil will be investigated. Primary oxidation and reduction processes, electron transfer, proton transfer and hydrogen atom transfer processes will be investigated in single crystals x-irradiated at liquid helium temperature. ESR and ENDOR spectroscopy will be the principal investigative tools. Crystals of host molecules containing a small percentage of radiation sensitizing molecules as quest molecules will be used to investigate radiation sensitization mechanisms. The overall objective of this research is to provide a rational basis for the design of improved drugs for use in radiation therapy.